Ultrasonic transducers are used in a wide variety of applications wherein it is desirable to view the interior of an object non-invasively. For example, in medical applications physicians use ultrasonic transducers to inspect the interior of a patient's body without making incisions or breaks in the patient's skin, thereby providing health and safety benefits to the patient. Accordingly, ultrasonic imaging equipment, including ultrasonic probes and associated image processing equipment, has found widespread medical use.
Ultrasonic probes provide a convenient and accurate way of gathering information about various structures of interest within a body being analyzed. In general, the various structures of interest have acoustic impedances that are different than an acoustic impedance of a medium of the body surrounding the structures. In operation, ultrasonic probes generate a signal of acoustic waves that is then acoustically coupled from the probe into the medium of the body so that the acoustic signal is transmitted into the body. As the acoustic signal propagates through the body, part of the signal is reflected by the various structures within the body and then received by the ultrasonic probe. By analyzing a relative temporal delay and intensity of the reflected acoustic waves received by the probe, a spaced relation of the various structures within the body and qualities related to the acoustic impedance of the structures can be extrapolated from the reflected signal.
For example, medical ultrasonic probes provide a convenient and accurate way for a physician to collect imaging data of heart tissue or fetal tissue structures within a body of a patient. In general, the heart or fetal tissues of interest have acoustic impedances that are different than an acoustic impedance of a fluid medium of the body surrounding the tissue structures. In operation, such a medical probe generates a signal of acoustic waves that is then acoustically coupled from a front portion of the probe into the medium of the patient's body, so that the signal is transmitted into the patient's body. Typically, this acoustic coupling is achieved by pressing the front portion of the probe into contact with a surface of the abdomen of the patient.
As the acoustic signal propagates through the patient's body, portions of the signal are weakly reflected by the various tissue structures within the body and received by the front portion of the ultrasonic medical probe. As the weakly reflected acoustic waves propagate through the probe, they are electrically sensed by electrodes coupled thereto. By analyzing a relative temporal delay and intensity of the weakly reflected waves received by the medical probe, imaging system components that are electrically coupled to the electrodes extrapolate an image from the weakly reflected waves to illustrate spaced relation of the various tissue structures within the patient's body and qualities related to the acoustic impedance of the tissue structures. The physician views the extrapolated image on a display device coupled to the imaging system.
Since the acoustic signal is only weakly reflected by the tissue structures of interest, it is important to try to provide efficient acoustic coupling between the front portion of probe and the medium of the patient's body. Such efficient acoustic coupling would insure that strength of the acoustic signal generated by the probe is not excessively diminished as the signal is transmitted from the front portion of the probe into the medium of the body. Additionally, such efficient acoustic coupling would insure that strength of the weakly reflected signal is not excessively diminished as the reflected signal is received by the front portion of the probe from the medium of the body. Furthermore, such efficient acoustic coupling would enhance operational performance of the probe by reducing undesired reverberation of reflected acoustic signals within the probe.
An impediment to efficient acoustic coupling is an acoustic impedance mis-match between an acoustic impedance of piezoelectric materials of the probe and an acoustic impedance of the medium under examination by the probe. For example, one piezoelectric material typically used in ultrasonic probes is lead zirconate titanate, which has an acoustic impedance of approximately 33 * 10.sup.6 kilograms/meter .sup.2 second, kg/m .sup.2 s. The acoustic impedance of lead zirconate titanate is poorly matched with an acoustic impedance of human tissue, which has a value of approximately 1.5 * 10.sup.6 kg/m .sup.2 s.
Furthermore, since the human body is not acoustically homogeneous, different frequencies of operation of an ultrasonic probe are desirable, depending upon which structures of the human body are serving as an acoustic transmission medium and which structures are the target to be imaged. Many commercially available ultrasonic probes include a transducer array that is optimized for use at only one particular acoustic frequency. Accordingly, when differing applications require the use of different ultrasonic frequencies, a user typically selects a probe which operates at or near a desired frequency from a collection of different probes. Complexity and cost of the ultrasonic imaging equipment is increased because a variety of probes, each having a different operating frequency, is needed. An economical and reliable alternative to manually coupling different transducers to such imaging systems is needed. Automated electrical switching systems have been explored but they have been too costly and complex to provide efficient electrical coupling of probe control lines to imaging system components.
Previously known dual frequency ultrasonic transducers utilize a transducer with a relatively broad resonance peak. Desired frequencies are selected by filtering. Current commercially available dual frequency transducers typically have limited bandwidth ratios, such as 2.0/2.5 MHz or 2.7/3.5 MHz. Graded frequency ultrasonic sensors that compensate for frequency downshifting in the body are disclosed in U.S. Pat. No. 5,025,790, issued Jun. 25, 1991 to Dias. Dual frequency ultrasonic transducers can additionally provide for added flexibility in "color flow" mapping wherein a first frequency is used for conventional echo-amplitude imaging and a second frequency is used for doppler shifted flow imaging.
Probes currently in use, such as those mentioned above, typically include an acoustic impedance matching layer adhesively bonded to the transducer for improving acoustic coupling between the transducer and an object under examination, such as human tissue. The layer matches the acoustic impedance of the transducer to the acoustic impedance of human tissue. However, such previously known acoustic coupling improvement schemes have had only limited success and have created additional manufacturing, reliability and performance difficulties. For example, many previously known impedance matching layers are frequency selective, so as to correctly match the transducer impedance to the impedance of human tissue only over a narrow band of frequencies. Therefore, such previously known impedance matching layers act as filters, further limiting usable bandwidth of a probe.
Furthermore, any unnecessary adhesive bonding should be minimized. Manufacturing difficulties are created by adhesive bonding processes used to implement previously known impedance matching schemes. For example, care must be taken to insure that no voids or air pockets are introduced to any adhesive layer that would impair operation of the probe. Additionally, if the adhesive layer is not acoustically transparent, operational performance is limited at higher acoustic signal frequencies, such as frequencies above 20 megahertz.
What is needed is a tunable ultrasonic probe that provides efficient electrical coupling to imaging system components, while further providing efficient acoustic coupling to the desired medium under examination by the probe.